1. Field of the Invention
The invention relates to routing in mobile communication systems. Particularly, the invention relates to the routing of communications to a Voice over IP (VoIP) terminal in a mobile communication system.
2. Description of the Related Art
Recently Wireless Local Area Networks (WLAN) have become important in mobile communications. The advantage of WLANs over licensed band cellular communication systems such as the Universal Mobile Telecommunication system (UMTS) and Global System of Mobile communications (GSM) lies in the facts that they use an unlicensed band and the cell sizes are much smaller. These facts make possible to build private WLANs operated by small corporate entities and individual users. The cost of wireless communication in these WLANs is significantly cheaper than in licensed band cellular systems. WLANs have mostly been used for Internet access, but the idea of providing voice communications over WLANs has recently gained momentum. In order to obtain a wide market share for voice over WLAN technologies and to provide a reliable service experience for end-users, it is necessary to be able to provide dual system terminals, which support both WLAN and licensed band based radio access. In other words, it must be possible for users to roam in both WLANs and licensed band cellular systems. Usually WLAN radio access is used in urban areas where there exists a WLAN infrastructure, whereas licensed band cellular systems are used in areas outside the WLAN coverage.
3G Partnership Project has standardized the IP Multimedia Subsystem (IMS) in order to cater for VoIP and other IP based multimedia services. Typically, a UMTS Radio Access Network is used to access a core network, which supports the IMS. However, existing circuit switched core network infrastructures, which comprise Mobile Switching Centers (MSC), Home Location Registers (HLR), Visitor Location Centers (VLR), Camel Service Entities (CSE) and Service Control Points (SCP), provide a wide range of services. When operators wish to accommodate dual system terminals with both WLAN and licensed band radio access capabilities, it would be beneficial, if the operators had some mechanism of offering same services over both radio access technologies. Especially, the providing of backward compatible service is important. In other words, it is necessary to be able to provide familiar look-and-feel services from the licensed band cellular system also in the WLAN side. These services are referred to as the legacy services. Examples of such services include call forwarding, prepaid, premium rate and free service numbers, call waiting and call transfer. Usually, prepaid service and service numbers are provided using Intelligent Network infrastructure comprising MSCs and SCPs. In the 3GPP standardized version of Intelligent Networks the SCPs are referred to as the CSEs.
Reference is now made to FIG. 1, which illustrates the problems associated with the providing of legacy services for dual system terminals in prior art. FIG. 1 illustrates the fact that in practice the legacy services must be rebuilt in the IMS. In IMS the network elements and the protocols are largely different so this represents a significant effort. In Figure there is a Mobile Station (MS) 100, which is a dual system mobile station capable of communicating both over a WLAN radio access and a licensed band radio access. The licensed band radio access may be, for example, a Time Division Multiple Access (TDMA) based GSM radio access or a Wideband Code Division Multiple Access (WCDMA) based UMTS radio access. In FIG. 1 there is also a WLAN 124, which communicates with an IP Multimedia Subsystem (IMS) comprising at least a P-CSCF 102, an I-CSCF 104, an S-CSCF 106, a MGCF 120 and a MGW 122. Multimedia communications to and from MS 100 when in the area of WLAN 124 are provided via IMS. WLAN 124 is connected to Media Gateway (MGW) 122, which converts IP-based user plane traffic to circuit switched PSTN 126. WLAN 124 communicates also with Proxy Call State Control Function (P-CSCF) 102. Signaling plane traffic is routed to a P-CSCF such as P-CSCF 102. The signaling plane traffic is, for example, Session Initiation Protocol (SIP) based. SIP is defined in Internet Engineering Task Force (IETF) document RFC 3261. P-CSCF 102 is used to access Inquiring Call State Control Function (I-CSCF) 104, which determines using a Home Subscriber Server (HSS) 108 Serving Call State Control Function (S-CSCF) 106 in which a given subscriber is currently registered. The S-CSCF controls the multimedia communications originating from and terminating to MS 100. The S-CSCF communicates with Media Gateway Control Function (MGCF) 120, which converts signaling plane traffic into circuit switched signaling. For example, MGCF 120 converts the SIP signaling used between MS 100, P-CSCF 102, I-CSCF 104, S-CSCF 106 and MGCF 120 into the ISDN User Part (ISUP) signaling used in PSTN 126. MGCF 120 also controls MGW 122 using, for example, International Telecommunications Union (ITU-T) H.248 protocol. S-CSCF 106 is connected to three service platforms, namely Application Server (AS) 110, CSE 116 and Open Service Architecture (OSA) server 118. S-CSCF 106 is connected to CSE 116 via IP Mobility (IM) Service Switching Function (SSF) 112. S-CSCF 106 is connected to OSA server 118 via Service Capability Server (SCS) 114.
In FIG. 1 there is also a GSM/UMTS BSS 160, which is connected to a GSM/UMTS circuit switched core network comprising at least an MSC 150, a VLR 152, a GMSC 156, an HLR 154 and a CSE 158. GSM/UMTS BSS 160 is connected to MSC 150. MSC 150 comprises also a VLR 152. MSC 150 is connected to GMSC 156. There is also HLR 154, which stores subscriber data pertaining to the location of subscribers and their service data. GMSC 156 is also connected to PSTN 126. CSE 158 controls GMSC 156 and MSC 150 in the providing of IN services to the subscribers served by BSS 160. CSE 158 has also an interface to HLR 154, which allows the enquiring and modifying of service data in HLR 154. A plurality of standardized supplementary services is implemented directly by MSC 150, GMSC 156, VLR 152 and HLR 154. Examples of such services include call forwarding, call waiting, call transfer, call completion to busy subscriber, closed user group and call barring. In addition to these there may be a variety of vendor specific supplementary services implemented directly in these network elements. In order to cater for the aforementioned legacy supplementary services a variety of service functionalities are present in MSC 150, GMSC 156, VLR 152 and HLR 154. These service functionalities are illustrated in FIG. 1 as service functionality sets 170-174. Each service functionality set may comprise a number of different service functionalities hosted in a given network element.
In order to support the same legacy services while MS 100 is in the service area of WLAN 124, the service functionality sets 170-174 must be ported to corresponding IMS network elements comprising at least P-CSCF 102, I-CSCF 104, S-CSCF 106 and HSS 108. This represents a significant task since all the development effort put in the service functionality sets 170-174 must be repeated when equivalent service functionality sets 180-184 are implemented in IMS network elements. For example, service functionality set 170 in MSC would correspond to service functionality set 182 in S-CSCF 106 and service functionality set 171 in CSE would correspond to service functionality sets 181, 183 and 184 in AS 110, CSE 116 and OSA server 118, respectively. However, the correspondence is not direct and obvious. It is sufficient to say that the work in the porting of legacy service functionality sets from the GSM/UMTS circuit switched core network to IMS side is non-trivial since the protocols used between the IMS network elements and the MS 100 are largely different from the ones used in GSM/UMTS circuit switched core network.
One possibility in the providing of legacy services for mobile stations roaming from GSM/UMTS BSS to WLAN side is presented in publication “SIP-Enabled Gateway MSC: Linking WiFi Hot Spots with 2.5/3G Networks”, Amir Atai, Ajay Sahai, Telica, Mar. 31, 2004. The solution disclosed by Atai comprises the connecting of WLANs directly to a GMSC in the circuit switched core network, which acts also as a serving Visitor MSC (VMSC). The disadvantage of the solution disclosed by Atai is that a given subscriber is always served by a given GMSC. However, even in the case of dual system terminals, it must be possible for the operator to receive a terminating call for a given terminal in any GMSC. The treatment of terminating calls in the GMSC must be uniform across 2G/3G and WLAN terminals. The call must be routed to the correct serving VMSC using a roaming number obtained from an HLR irrespective of the type of the terminal. Further, it is beneficial to be able to configure the DNS so that a number of MSC servers are referred to using the same Fully Qualified Domain Name (FQDN), for example, “sip.operator.com”, wherein “operator” stands for the operator name and “sip” stands for a set of SIP registrars. When a dual system terminal registers to the circuit switched core network via a WLAN and provides the FQDN for the SIP service, it is possible for the DNS to return IP-addresses for different MSC servers acting as SIP registrars in a round-robin fashion. Thus, at different registration times a different IP address may be provided from the DNS to the dual system terminal. Additionally, some legacy services may require that calls pertaining to legacy services must be routed to/via a voice server or a centralized IN service switching point. Thus, it would be a benefit to be able to use legacy ISUP signaling between the circuit switched core network elements. When pure SIP signaling is used the users' ITU-T E.164 format subscriber numbers are not available.